1. Field of the Invention
The present invention relates to a bi-directional DC-DC converter for carrying out a step-up operation and a step-down operation.
2. Description of the Related Art
A recovery-less step-up chopper circuit is known to serve as a high-efficiency, low-noise power converter for a vehicle. FIG. 1 is a circuit diagram illustrating a recovery-less step-up chopper circuit according to a related art. In FIG. 1, an AC power source Vac is connected to a full-wave rectifying circuit B1. The circuit B1 rectifies an AC power source voltage from the AC power source Vac and outputs the rectified voltage to a positive output end P1 and a negative output end P2.
Between the positive and negative output ends P1 and P2 of the full-wave rectifying circuit B1, there is connected a first series circuit that includes a step-up winding 5a (whose number of turns being n1) of a step-up reactor L1, a winding 5b (whose number of turns being n2) of the reactor L1, a zero-current switching (ZCS) reactor L2, a diode D11, a smoothing capacitor C1, and a current detecting resistor Rd.
Between the positive and negative output ends P1 and P2 of the full-wave rectifying circuit B1, there is also connected a second series circuit that includes the step-up winding 5a of the step-up reactor L1, a switch Q1 that is a MOSFET, and the current detecting resistor Rd. A diode D12 is connected between the smoothing capacitor C1 and a connection point of the step-up winding 5a, the winding 5b, and the drain of the switch Q1.
The switch Q1 turns on/off under PWM control conducted by a control circuit 100. The diode D11 and smoothing capacitor C1 form a rectifying-smoothing circuit. The smoothing capacitor C1 is connected in parallel with a load RL, so that the smoothing capacitor C1 smoothes a rectified voltage from the diode D11 and provides the load RL with DC power. The current detecting resistor Rd detects an input current passing through the full-wave rectifying circuit B1. The control circuit 100 includes an error amplifier 111, a multiplier 112, an error amplifier 113, an oscillator (OSC) 114, and a PWM comparator 116.
The error amplifier 111 has a positive terminal (+) to receive a reference voltage E1 and a negative terminal (−) to receive the voltage of the smoothing capacitor C1. The error amplifier 111 amplifies an error between the voltage of the smoothing capacitor C1 and the reference voltage E1 and outputs an error voltage signal to the multiplier 112. The multiplier 112 multiplies the error voltage signal from the error amplifier 111 by the full-wave rectified voltage from the positive output end P1 of the full-wave rectifying circuit B1 and outputs a resultant voltage to a positive terminal (+) of the error amplifier 113.
The error amplifier 113 has the positive terminal (+) to receive the multiplication resultant voltage from the multiplier 112 and a negative terminal (−) to receive a voltage proportional to the input current detected by the current detecting resistor Rd. The error amplifier 113 amplifies an error between the voltage from the current detecting resistor Rd and the multiplication resultant voltage and outputs an error voltage signal as a feedback signal FB to the PWM comparator 116. The oscillator 114 generates a triangular wave signal having a predetermined period.
The PWM comparator 116 has a negative terminal (−) to receive the triangular wave signal from the oscillator 114 and a positive terminal (+) to receive the feedback signal FB from the error amplifier 113. The PWM comparator 116 generates a pulse signal that is ON if the value of the feedback signal FB is equal to or higher than the value of the triangular wave signal and OFF if the value of the feedback signal FB is lower than the value of the triangular wave signal and applies the pulse signal to the gate of the switch Q1.
Namely, the PWM comparator 116 provides the switch Q1 with a duty pulse that is dependent on the output of the error amplifier 113, i.e., the difference between the output of the current detecting resistor Rd and the output of the multiplier 112. This duty pulse is a pulse width control signal that has a predetermined period and continuously compensates for variations in the AC power source voltage and DC load voltage. In this way, the related art controls an AC power source current waveform so that it may agree with an AC power source voltage waveform, thereby improving a power factor.
Operation of the recovery-less step-up chopper circuit illustrated in FIG. 1 will be explained. When the switch Q1 is turned on, a voltage produced by rectifying the AC power source voltage passes a current in a clockwise direction through a path extending along Vac, B1, 5a, Q1, Rd, B1, and Vac. The current of the switch Q1 starts from zero, and therefore, the switch Q1 achieves a zero-current switching (ZCS) operation. A current passing through the diode D11 decreases to zero to turn off the diode D11. In a recovery time of the diode D11, it passes a spike current through the switch Q1. This spike current is restricted by the impedance of the ZCS reactor L2.
When the switch Q1 is turned off, energy accumulated in the step-up reactor L1 during the ON period of the switch Q1 passes a current through the step-up reactor L1. This current is not suddenly passed to the ZCS reactor L2. Namely, a differential current between the current passing through the step-up reactor L1 and a current passing through the ZCS reactor L2 is passed through the diode D12 to the smoothing capacitor C1, to supply power to the load RL. The current passed through the diode D12 linearly decreases.
The current passing through the ZCS reactor L2 due to the energy accumulated in the ZCS reactor L2 is passed through the diode D11 to the smoothing capacitor C1, to supply power to the load RL. The current passing through the diode D11 linearly increases. When the current passing through the ZCS reactor L2 becomes equal to the current passing through the step-up reactor L1, the current passed through the diode D12 becomes zero.
When the switch Q1 is turned on, the current passing through the ZCS reactor L2 linearly decreases to zero to turn off the diode D11. As the current passing through the ZCS reactor L2 decreases, the current passed to the switch Q1 increases. When the current passed to the switch Q1 becomes equal to the current passing through the step-up reactor L1, the current passing through the ZCS reactor L2 becomes zero. Consequently, a zero-current switching operation is achieved.
In this way, a zero-current switching operation is carried out when the switch Q1 is turned on, to reduce a loss caused by the recovery of the rectifying diode D11 and make current changes gradual to achieve a high-efficient, low-noise switching operation.